Factual error: Grain engulfment is not depicted in a scientific manner. It is not possible for engulfment to happen without a valve being opened at the base - which has not been shown in the movie when the boy falls in the grain storage. It is very unlikely that two engulfments will happen so close by in the same storage - if so, they will converge quickly. But, the movie shows both victims help each other one by one. They succeed in 5-10 seconds. Engulfment become entrapment in less than 20 seconds and require more than thrice the body weight to pull some one out (unlike one kid being able to pull other one out while both are on the grain). Finally, they both sit on the door and avoid entrapment.
Factual error: Emily Blunt takes her blood pressure manually, pumping up a cuff but not listening to her pulse with a stethoscope to get the reading. Not possible without a new style automatic BP machine which would have made noise.
Factual error: Several shotgun mistakes in the final scene: 1) After firing the shotgun inside the room there should have been some smoke in the air. 2) She just shot the alien, so when she pumps the shotgun again it should eject an empty shell, but it does not. 3) The sound a loaded shotgun makes when you pump it is distinctly different from the sound an empty shotgun makes. It sounds like her shotgun is empty. 4) When the alien enters the cellar, the pump of the shotgun is pulled back. You never hear it racked forward before it is fired.
Factual error: The son's death happened in 2020 and it was day 89. On day 473, the mother writes in the calendar and it's Wednesday 3 October. So, the year must be 2021. But 3 October 2021 is not a Wednesday, but rather a Sunday. (00:26:00)
Factual error: The kids take shelter in a truck and whilst the monster is distracted they release the brakes and it starts rolling down a hill. The brakes would have been seized up due to lack of use so the kids wouldn't have been able to release them in the first place.